This invention relates to a method for positioning superheaters in biomass burning steam generators for minimizing the corrosion of superheater surfaces at a high temperature.
This invention relates also to a steam generator using biomass as a fuel, comprising a combustion chamber, a flue gas duct, a steam circuit, and superheaters positioned for minimizing corrosion at a high temperature.
The use of fossil fuels in energy production will be increasingly replaced in the future by the use of renewable natural resources. The use of various biomasses, such as wood, straw, willow, black liquor, etc., as a source of energy would reduce CO2 discharges as compared to fossil fuels. However, in the process of using more and more biomasses as a fuel, it has been discovered that the heat transfer surfaces of steam generators contaminate and corrode prematurely in response to hot flue gases with biomass as a fuel. It has been contemplated that alkali metal sulphates, and especially alkali metal chlorides, constitute a cause for the accelerated contamination and corrosion rate of heat transfer surfaces. It has been contemplated that alkali, sulphur, and chloride, present in a fuel, discharges from the fuel in at least partially gaseous alkali sulphate and chloride in the combustion chamber of an incineration plant and condensates then on colder heat transfer surfaces in a superheater region, resulting in excessive contamination and corrosion.
It is prior known that the power generation efficiency of a power plant generator is the higher the higher the pressure, and thus temperature, applied to water vapour fed into a steam turbine. Hence, it is preferred that steam be superheated to become as hot as possible, yet without risking the endurance of a power plant. All-coal fired generator plants reach a steam temperature of more than 550xc2x0 C. without notable corrosion hazards in a superheater region. In the process of burning biomasses, major corrosion has been discovered at superheating temperatures as low as less than 500xc2x0 C.
Mixed burning has been found to have a positive effect on the endurance of a steam generator in the process of burning biomasses. The burning of 30% or more of e.g. peat along with biomass has resulted in a remarkable mitigation of the corrosion and contamination problem. This has been explained by the fact that KCl, splitting from biomass and present in a gas phase, reacts with SO2 contained in flue gases to form K2SO4 which, by virtue of its higher melting point, does not form a molten phase on superheater surfaces and, consequently, does not smudge or corrode the superheater surfaces as much as KCl.
Since mixed burning increases carbon dioxide emissions and since even partial use of fossil fuels is not advisable in view of CO2 discharges, the use of peat is disallowed e.g. in Sweden even for reasons relating to incineration technology. It is obvious that increasingly stringent environmental regulations and public disapproval shall make the use of peat and coal in mixed burning more difficult in other countries, as well.
This invention enables the achievement of high steam values (high temperature and pressure) in all-biomass burning facilities without increasing the corrosion and fouling hazard for the superheater surfaces of a steam generator.
In order to accomplish this, a steam generator and a method of the invention are characterized by what is defined in claims 1 and 3. Preferred embodiments of the invention are set forth in the dependent claims 2 and 4. Background of the present invention will be discussed next and the invention itself will be described later with reference to the accompanying FIG. 1.